This invention relates to a frequency shift keyed demodulation system and, more particularly, to a system for demodulating continuous phase frequency shift keyed signals by recursively estimating the symbol values in a sequence.
Upon reception, a continuous phase frequency shift keyed signal is typically corrupted by white Gaussian noise and non-white interference. Discrete time samples of the sum of the signal of interest, noise and interference are produced by band limiting the received wave form, sampling at a rate satisfying the Nyquist criterion, complex heterodyning and filtering to pass positive frequency components and eliminate negative frequency components. In its simplest form, a demodulator can examine the received signal and choose as its output symbol that symbol which most closely corresponds with the received signal, independent of the value of any other previously received symbol. At the other extreme, a demodulator can examine every possible combination of symbols in a sequence and determine which combination most closely corresponds to the received multisymbol signal. The first of these demodulation techniques is relatively simple to implement but suffers from a relatively high symbol error rate. The other of the techniques has a low symbol error rate but is impractical to implement. It is therefore an object of the present invention to provide a demodulator for frequency shift keyed signals which results in a low symbol error rate and is practical to implement.
The most commonly utilized type of frequency shift keying is continuous phase frequency shift keying. The fact that the phase is continuous between adjacent symbols provides useful information. It is therefore another object of this invention to provide a demodulator which utilizes this continuous phase information.